Carbon fibers are high-strength light-weight materials commonly produced by heat treatment and pyrolysis of polyacrylonitrile, a synthetic material made from petroleum feedstock but other precursors are also used to a minor extent such as petroleum- or coal-based pitch and rayon fibers. There are certain drawbacks in the current precursors such as the high price of polyacrylonitrile and its slow graphitization and the uneven quality of pitch. In addition, the two major commercial precursors used are from non-renewable sources.
Lignin is present in all vascular plants making it second to cellulose in abundance among polymers in nature. In the pulp and paper industry, large quantities of lignin are produced as a byproduct with primary use as the source of internally generated energy in pulp mills. The kraft process is predominant in the world for liberating cellulosic fibers from wood for further processing to paper, board and tissue products. In the process, lignin becomes dissolved in the alkali pulping liquor, denoted black liquor, from where it can be further processed to energy by combustion of the partly evaporated black liquor or, alternatively, isolated in solid form by addition of acid. The isolation of lignin may occur in several steps since a major portion of lignin can precipitate out from the black liquor already at high pH-values as described in the book Lignins (Eds K. V. Sarkanen and C. H. Ludwig, Wiley-Interscience 1971, p 672). Such lignin precipitate will still contain appreciable amounts of sodium and other inorganic species making the lignin unsuitable as precursor for structural carbon fibers (see below).
Alkali lignins are obtained from black liquors obtained from either kraft or soda pulping. Commercially, these pulping processes are applied on softwoods, hardwoods as well as on annual plant biomass. On pulping, some of the wood polymers, notably lignin and hemicelluloses, are to a major extent chemically modified and solubilized in the black liquor.
Among wood species used in alkali pulping processes major gymnosperm species (softwood) include spruce, pine, larch, hemlock and Douglas fir. Major angiosperm species (hardwood) include birch, aspen, poplar, eucalypt species, acacia, and maple.
In the published literature, it has been suggested that lignin might be an alternative precursor of carbon fiber due to its potentially large availability, its expected lower cost, and its high content of carbon (>60%). In addition, lignin is a renewable material. Two types of carbon fibers have been discerned; 1) continuously spun, solid and homogeneous carbon fibers used as strength-giving reinforcement elements in construction materials (herein also referred to as structural carbon fibers) and 2) activated porous carbon fibers with large internal pore structure for adsorption of gases and liquids where the activation can be done chemically with e.g. potassium or sodium hydroxide, zinc chloride or phosphorous acid, or physically with e.g. steam or carbon dioxide, or by applying the latter to chemically pre-activated fibers (Carbon Fiber Application, in the 3rd ed. of the book Carbon Fiber, Eds. Donnet, Wang, Rebouillat and Peng, Marcel Dekker 1998, p. 463).
In an early attempt to carbonize lignin fibers using lignin originating from woody material, several types of activated carbon fibers suitable for adsorbing products were produced as described in U.S. Pat. No. 3,461,082. Either thiolignin (kraft lignin), alkali lignin (from soda pulping), or calcium lignosulfonate from hardwood and softwood were used and in the examples, fibers produced using wet spinning, dry spinning and melt spinning, are described. Although dry spinning appears to be the preferred mode of fiber production, in Example 5,a mixture of softwood and hardwood thiolignin (1:1 by weight) was used in argon atmosphere at 170° C. to make lignin fiber by melt spinning. After pretreatment in air at 150° C. for 10 hours, the fibers were further heated to 900° C. and activated at that temperature during 1 hour by introduction of air. In further examples, other activating agents such as zinc chloride, sodium hydroxide, or potassium hydroxide were tried. However, only short-length fibers could be produced.
To date, all attempts to produce continuous carbon fibers from 100% unfractionated or fractionated softwood lignin have failed. Only discontinuous lignin fiber production has been possible, by the use of low molecular mass fraction lignin, obtained from fractionation of such lignin in organic solvent.
Fibers from extensively purified hardwood kraft lignin, on the other hand, have been made by extrusion of the lignin after admixing with softening agents such as poly-ethyleneterephtalate (PET) or poly-ethyleneoxide (PEO). The resulting lignin fiber has been further converted into carbon fiber through stabilization in air and carbonization.
Since softwood pulping is predominant in the northern hemisphere, there is a need for a method making use of this source of raw material by producing lignin fibers from softwood alkali lignin, for further use as precursor for carbon fiber manufacture.
Moreover, there exists a need for a method of manufacturing lignin fibers, for subsequent use as carbon fiber precursor, from hardwood alkali lignin, without the need for expensive softening agents and elaborate processes for purification of the hardwood alkali lignin.